XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

Pre-HE and HE power?

 
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Old 05-10-2019, 02:41 AM
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Default Pre-HE and HE power?

Hello, I'm new to posting on this forum, I had some questions about HE and pre-HE v12's. I am not sure if this is the right section to post in. Ok so I am planning on building a 5.3 with a 5 -speed for a series 1 XJ6 swap, I don't need crazy power, but I would like to see around 300-320hp at the wheels , My question is, do you think it is worth trying to find a Pre-HE motor due to their potential to make more power, or can my power figures be achieved just as easily with the HE motor? Some details about what I plan to do, I am going to be running an MS2 V3.0 controlling fuel with bank fire and maybe ignition on wasted spark, I plan on putting either a Getrag or a Tremec 5-speed behind it. I am swapping it into a series 1 XJ6. Again sorry if this is not in the right section, I figured that there are more v12 XJS owners than XJ12.
 
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Old 05-10-2019, 03:26 AM
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It depends on what you want the car to do. Do you want it do 0 - 100km/h quickly or do you want to cruise at 300km/h

If it's 0 - 100km/h you want low down torque and the HE makes more torque lower i the RPM band. The best you will get from a NA HE 5.3L is around 350hp.And will require head, intake and exhaust work.

On the other hand if you want to rev the car and make 400fwhp at 7500rpm the pre-HE, this is also doable with head, intake and exhaust mods.

There is only 2 ways to make power from a NA Jag V12 and that is to increase capacity and/or rev it. Or forced induction

Other will chime in, with both similar and other views.
 
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Old 05-10-2019, 06:10 AM
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An HE motor depends a bit on the specs. USA spec motors made about 260 BHP DIN (ie at the flywheel engine in production spec). UK and RoW motors made more like 295. BUT, by the time it was through the GM400 to the rear wheels think more like 210UK and 180 USA actually at the wheels. A manual box will lose far less power so think with a UK spec HE in top nick about 240 at the wheels. This will make a Series I XJ saloon car very fast compared with the original.

After that, the cost for extra RWHP is pretty prohibitive, with may be another 20 BHP relatively easily available with bolt-on intake mods, exhaust and ECU mods (see AJ6 Engineering's website for those things).
 

Last edited by Greg in France; 05-10-2019 at 02:48 PM.
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Old 05-10-2019, 12:49 PM
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For normal road use the HE and pre HE will make similar amounts of power, but the pre HE will use a lot more fuel to do it. 400hp is generally considered the upper limit of the HE due to head design. If you are racing the car and will be driving at full throttle most of the time, go pre HE, otherwise go HE.
 
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Old 05-10-2019, 01:45 PM
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Cool, thanks for the info! I don't mind the bad mileage, this won't really be a road trip car. Thays what my '83 XJ is for, I probably won't ever race it but I do want to rev it out to 7500, for that amazing (imho) sound! in all honesty With the manual swap and custom EFI tuning, i would be pretty happy with 250-270 rwhp, of course, it would be nice to get more but i do realize that it gets very expensive.
 

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Old 05-10-2019, 05:15 PM
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Good luck , many have tried to make an honest 400hp at the flywheel,very few have done it! my preference would be to make more torque in mid range than 400hp at 8000revs!
while your trying to get going , i'd be long gone with my torque and you would still trying to get your revs up! been there done that!



that HP at the wheels maybe 350hp!

and in this modern car world ,just about all are using forced induction,(as has been said)!

stop and think about it , would you try to suck the air in or force the air in, forcing in would reduce a lot of air flow restrictions!
 
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Old 05-10-2019, 05:17 PM
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I was/am lookong into some of this stuff, easily several thousand for better intake manifolds and a modern v12 ECU with CoP ignition. Not worth it at the moment.

For noticable gains and better sound open up the air intakes, wrap the intake manifolds in heat shielding, delete CATS and center mufflers, toss in an Xpipe if you feel like it and the car will move faster and sound much better.

I cut out the trumpet and face of the filter box and attached a mesh with K&N filters, it works nicely to scoop and ram the air back. If I ever get on a dyno I'll post the results but it is definately faster.
 
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Old 05-10-2019, 08:53 PM
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The ONLY way to increase power under 5250rpm is to increase torque. This can only be done in 2 ways increase capacity or volumetric efficiency.

A 6.0L V12 is a massive improvement over the 5.3L. Testing by TRW a totally stock HE 5.3L made 285hp at ≈5250rpm this is a complete engine with full exhaust stock air boxes and paper filters. This engine was then configured with an 80mm stroke crank for 6.1L and power rose to 340hp (19% increase) @ 5000rpm but hp was up 27% at 3500rpm.

This testing shows that the intake manifold runners are NOT a restriction for 5.3L. I have confirmed this on a flow bench.

TWR also experimented with intake and exhaust manifolds off the GrpA XJS and this made a a huge increase in peak power but the increase was all over 3750rpm.

In a nutshell the cheapest way to increase power is buy a 6.0L V12 engine. To make a 6.0L from a 5.3L it can be fitted with large liners but will need custom pistons. A 6.0L fitted with large liners can use Chev pistons fore 6.7L.
 
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Old 05-10-2019, 11:59 PM
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Originally Posted by warrjon View Post

In a nutshell the cheapest way to increase power is buy a 6.0L V12 engine.
Or in this application where the OP is starting with a clean sheet, buying an XJR engine would probably be cheaper and easier.
 
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Old 05-11-2019, 01:47 AM
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Originally Posted by Jagboi64 View Post
Or in this application where the OP is starting with a clean sheet, buying an XJR engine would probably be cheaper and easier.
Not here in Australia, my 6.7L including the cost of the original 6.0L will be cheaper than buying an XJR 4.0L I6 if you can find one. This may be different in the US and UK.
 
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Old 05-11-2019, 07:14 PM
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I'm in Canada and I bought a running but rusty 1995 XJR for $1,000 with 130,000km on it. I can't get much done at a machine shop for that, certainly not an engine rebuild. The CAD-AUS dollars are about par now.
 
 
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